To improve high temperature stability and to give better physical and electrical properties over amine cured epoxy resin systems, it has been the general practice in the epoxy technological field to use anhydride curing agents with epoxy resins, particularly for high voltage insulation applications. Most epoxy-anhydride formulations require elevated-temperature cures, and for most commercial applications it is necessary to add some material to speed the rate of cure. Consequently, a considerable amount of effort has been devoted in recent years to develop a perfect catalyst or accelerator for curing epoxy resins, especially those used for high voltage coil insulation, i.e., over about 7,000 volts. In high voltage coils, only an absolute minimum of voids can be tolerated in the resinous insulation. Therefore, the applied resin impregnating composition must be extremely fluid, solventless, and capable of a very fast gel, so that resin will not easily drain from a coil during curing.
The properties desired of such a catalyst or accelerator are: it should be inexpensive and readily available; it should gel the epoxy resin system at times below 50 minutes and preferably below 35 minutes at about 135.degree. C. to 175.degree. C.; it should be completely soluble with the epoxy resin-anhydride system at all temperatures; the initial viscosity of the catalyzed resin system should be below about 350 cps. at 25.degree. C.; the storage life of the catalyzed resin system should be over at least 80 days and preferably about 180 days at 25.degree. C., i.e., the viscosity should remain below about 1,000 cps. during that period; it should not adversely affect the mechanical properties of the cured resin system; after cure, the resin system should have power factor values of below about 30% at 150.degree. C.
Several latent catalysts have appeared on the commercial scene in recent years. Included are quaternary ammonium halides such as benzyltrimethyl-ammonium chloride, stannous octoate, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. However, all of these materials failed to meet all of the above described requirements and have been rejected.
Smith, in U.S. Pat. No. 3,784,583, taught the use of a quaternary organic phosphonium salt, as a latent catalyst, for a solventless, highly fluid, resinous, epoxy-anhydride impregnating composition. While the phosphonium salt combined superior gel times with excellent pot life, i.e., good latent catalytic activity, and good electrical properties, the latent catalyst was not readily available.
Starck et al, in U.S. Pat. No. 2,801,228, used from 5 to 40 wt.% of metallic salts of enol-keto tautomers including nickel acetylacetone, as a curing-hardener substitute for amines and anhydrides, in epoxy resins. Naps, in U.S. Pat. No. 2,876,208, used from 0.1 to 20 wt.% of copper acetylacetone, as a stabilizer, in phenolic-epoxy resin systems.
Markovitz, in U.S. Pat. No. 3,812,214, taught catalytic cures of relatively viscous, epoxy resin systems, having initial viscosities of about 375 cps. to 1,200 cps. at 25.degree. C., by using up to 20 wt.% of a combination phenolic resin accelerator-metal acetylacetone catalytic hardener. Markovitz eliminated anhydride curing agents, and substituted phenolic resin accelerators and the use of up to 5 wt.% of metallic acetylacetonates. The useful acetylacetonates could contain essentially any metal anion, were readily available commercially, and were characterized as providing a superior pot life over systems containing anhydrides or boron trifluoride amine curing agents.
Epoxy-anhydride systems are still very useful. What is needed is an improved epoxy resin system, which still contains anhydride curing agents, but which will have good pot life and electrical properties. The epoxy-anhydride system should be very fluid, i.e., initially below about 350 cps. at 25.degree. C., to ensure its usefulness as an impregnating composition for high voltage applications, where complete coil impregnation is critical.